The present invention relates to ceramic decalcomania (decals) and particularly to improvements in decal structure which impart improved chemical durability to decorations on ceramic ware.
Overglaze decorations provided by firing fusible ceramic pigments onto previously glazed ceramic vessels such as cookware and dinnerware are repeatedly exposed to alkaline detergents in use. The need to protect such decorations from detergent attack has long been recognized; hence, present-day overglaze decorations often comprise a glass barrier layer over the pigment-containing design layer to protect the design from deterioration.
U.S. Pat. No. 2,734,840 to Kane, for example, describes a lithographic decal which includes a design layer consisting of a pigment pattern (the pigment being a ceramic pigment and including a minor amount of fusible fritted glass as a bonding agent), and a superimposed layer of a powdered, low-melting-point lead borosilicate glass. During firing, the powdered glass forms a protective layer over the non-durable pigments which is intended to protect the pigment from subsequent chemical attack.
As illustrated in U.S. Pat. No. 3,769,055 to Blanco, the manufacture of printed decals can also be accomplished by applying glass-free ceramic pigments to a decal base to form a design layer, and separately applying a layer of powdered lead borosilicate glass to the decal. On firing, this glass bonds the pigment to the ceramic article and acts to protect the pigment from subsequent damage.
U.S. Pat. No. 3,857,746 to Blanco et al. describes a ceramic decal produced by a four-color printing process wherein the colors are sequentially printed onto a decal base using mixtures of conventional ceramic pigments with a powdered CdO-containing lead silicate glass. The resulting design layer may optionally be covered with powdered CdO-containing glass for protection. Reportedly, this decal may be fired onto ceramic ware without the color deterioration normally accompanying the firing-on of multi-colored decorations.
In the manufacture of decalcomania for ceramic decorating, high concentrations of ceramic pigments such as CdS, CdS.Se, Co.sub.3 O.sub.4 and the like are typically added to the pigment-frit mixtures used to form the decal design layer. High color concentrations are used in order to minimize color dilution and provide intensely colored designs. At the same time, the thickness of the decal must be limited to assure proper adherence between the fired decoration and the ceramic article. These demands for low thickness and saturated color tend to limit the amount of fusible glass material which can be incorporated into the decal for chemical and physical protection.
Presently available decals such as above described, comprising lead borosilicate glass for protection, do not always provide decorations exhibiting a satisfactory degree of resistance to attack by alkaline detergents. Moreover, although the lead and cadmium retention of the decoration may initially be satisfactory, deterioration from detergent attack typically leads to increased lead and cadmium release. Detergent durability appears to suffer most when cadmium-based colors such as, for example, CdS yellow or CdS.Se (cadmium sulfoselenide) red ceramic pigments are present in the design layer.
It is quite difficult to effect improvements in decoration durability simply by modifying the composition of the protective layer glass; the composition of this glass is largely dictated by the characteristics of presently available ceramic pigments. Hence, a CdO-containing glass is required in some cases in order to insure compatibility among the pigments at decal firing temperatures, and a low-melting-point glass is needed to avoid color deterioration which would result if high decal firing temperatures were employed.
In any event, presently utilized protective layer glasses, by themselves, exhibit adequate detergent durability, and there is no indication that the use of more durable glass would solve the present problem. Although very durable enamel glasses have recently been developed, as illustrated, for example, by the enamel glasses described in recently issued U.S. Pat. No. 4,038,448 to Boyd et al., there is no evidence that such glasses would be any more compatible with common decal pigments than presently available protective layer glasses.
It is a principal object of the present invention to provide a ceramic overglaze decalcomania of novel structure which produces decorations exhibiting improved detergent durability while retaining the advantages of design flexibility, vivid color and handling convenience offered by prior art decalcomania.
It is a further object of the invention to provide improvements in decal structure which result in improved decoration durability without the need to develop new, more stable ceramic pigment systems.
Other objects and advantages of the invention will become apparent from the following description.